To understand a bit the rationale behind this bug called "heartbleed" we first need to understand some other concepts.
First, SSL is an encryption technology used to protect privacy of web users while they transmit information over the internet. It was first introduced by Netscape in 1994.
There are several implementations of this SSL protocol. One of these implementations is the popular library OpenSSL. It also implements TLS, an newer version of SSL. The implementation TLS provided by OpenSSL is buggy and hence vulnerable to attacks.
As this bug is in its heartbeat mechanism, they named it heartbleed bug.
What services are affected?
The affected component is the TLS implementation provided by OpenSSL. HTTPS servers that use OpenSSL are affected, as the HTTPS implementation uses the buggy TLS implementation (and its buggy heartbeat extension).
OpenSSH also uses OpenSSL (mainly for key generation functions), but not the buggy component (TLS implementation) so it is not affected. So there is no need to worry about SSH being compromised, though it is still a good idea to update openssl to 1.0.1g or 1.0.2-beta2 (but you don't have to worry about replacing SSH keypairs).
What are the OpenSSL affected versions?
OpenSSL 1.0.1 through 1.0.1f (inclusive) are vulnerable.
OpenSSL 1.0.1g is NOT vulnerable.
OpenSSL 1.0.0 branch is NOT vulnerable.
OpenSSL 0.9.8 branch is NOT vulnerable.
What is the estimated amount of affected servers?
The two most popular web servers, Apache and nginx use OpenSSL. Together, these vulnerable servers account for about two-thirds of the sites on the web.
How does it impact ProActive?
It does not impact ProActive as it does not depend on such implementation of SSL at all.
However we often see our web portals exposed through nginx to beautify the URLs where the portals are exposed. In such cases (knowing that nginx uses OpenSSL) we encourage the sysadmin to check if the version of the OpenSSL library installed is affected by the bug, and in the affirmative case, to upgrade it, renew all HTTPS certificates, and ask the users to renew their passwords.
Are there available tests?
Yes, there are a couple.